The exposure to environmental pollution could be linked to an increase in the prevalence of psychiatric disorders, according to a new study by a group of international researchers.

The effects of the environment on psychiatric and neurological conditions are increasingly being studied by researchers, motivated by emerging evidence from environmental events like the record-breaking smog that choked New Delhi two years ago.

In a new study published in the journal PLoS Biology, experts discovered that poor air quality was associated with higher rates of bipolar disorder and major depression in both US and Danish populations. The trend appeared even stronger in Denmark than in the US.

“Our study shows that living in polluted areas, especially early on in life, is predictive of mental disorders in both the United States and Denmark,” said computational biologist Atif Khan. “The physical environment warrants more research to better understand how our environment is contributing to neurological and psychiatric disorders.”

Despite the fact that mental illnesses develop due to a complex interplay of genetic predispositions and life experiences or exposures, genetics alone does not account entirely for variations in mental health. Researchers have long suspected that environmental factors also interact at different levels, and this new study highlights the importance of pollution in the larger equation.

To quantify air pollution exposure among individuals in the US, the researchers relied on the US Environmental Protection Agency’s measurements of 87 air quality measurements. For individuals in Denmark, they used a national pollution register that tracked a smaller number of pollutants with much higher spatial resolution.

Then, the team examined two population data sets. The first was a US health insurance claims database that included 11 years of claims for 151 million individuals. The second one consisted of all 1.4 million individuals born in Denmark from 1979 through 2002 who were alive and residing in Denmark on their tenth birthday.

Because Danes are assigned unique identification numbers that can link information from various national registries, the researchers were able to estimate exposure to air pollution at the individual level during the first ten years of their life. In the US study, exposure measurements were limited to the county level.

The findings of the researched created controversy and resistance among reviewers. The divided opinions of the expert reviewers prompted PLOS Biology to commission a special companion article from Prof. John Ioannidis of Stanford University.

“A causal association of air pollution with mental diseases is an intriguing possibility. Despite analyses involving large datasets, the available evidence has substantial shortcomings and a long series of potential biases may invalidate the observed associations,” says Ioannidis in his commentary. “More analyses by multiple investigators, including contrarians, are necessary.”

The significant associations between air pollution and psychiatric disorders discovered in the study do not necessarily mean causation, according to the researchers who said that further work is needed to assess whether any neuroinflammatory impacts of air pollution share common pathways with other stress-induced conditions. However, the findings warrant further research, to better understand the relationship between pollution and psychiatric disorders.